1. Field of the Invention
The present invention relates to a sensor IC (integrated circuit) device and an image sensor incorporating such a sensor IC device for use in facsimile machines, image scanners, and others for image reading purposes.
2. Description of the Prior Art
A conventional sensor IC device and a contact image sensor (hereafter also referred to as a CIS) incorporating it will be described with reference to FIGS. 2 and 3. As shown in FIG. 2, a conventional CIS is composed of a plurality of sensor IC devices 21. Each sensor IC device 21 includes an array of, for example, 96 light-sensing elements 23 such as phototransistors. Each light-sensing element 23 corresponds to one pixel.
By the side of each sensor IC device 21, an LED (light-emitting diode) 20 is provided that illuminates with light a paper sheet or other having an image to be read formed on it. Thus, the light-sensing elements 23 output electric signals in accordance with the light reflected from the paper sheet or other. The sensor IC device 21 successively outputs the signal from one light-sensing element 23 after another. Since the signal outputted from the sensor IC device 21 is feeble, it is then amplified by an amplifier 22 that is provided separately from the sensor IC devices 21. This operation is repeated to read the entire image on the paper sheet or other.
On the other hand, as shown in FIG. 3, in another type of conventional CIS, each sensor IC device 24 incorporates an amplifier circuit 25. Accordingly, the sensor IC device 24 outputs the signal from each light-sensing element 23 after amplifying the signal with its own amplifier circuit 25, and thus the signal outputted from the sensor IC device 24 is used intact as the output of the CIS. Note that, in FIG. 3, such components as are found also in FIG. 2 are identified with the same reference numerals, and overlapping explanations will not be repeated.
However, the CIS shown in FIG. 2 requires an amplifier 22 as a separate component. Quite inconveniently, this requires not only extra space to mount the amplifier, but also extra wiring for it, and thus leads to reduced reliability and to increased cost. On the other hand, the CIS shown in FIG. 3 suffers from variation in "dark output voltage" (that is, the voltage that the CIS outputs when it receives no reflected light), because the offset voltage of the amplifier circuit 25 incorporated in each sensor IC device 24 varies slightly from one sensor IC device to another. Quite inconveniently, making the dark output voltage constant requires extra measures, such as trimming, that are difficult to realize.